Stressed Source/Drain CMOS and Method of Forming Same

ABSTRACT

A complementary metal-oxide semiconductor (CMOS) structure includes a substrate and a P-type field effect transistor (FET) and an N-type FET disposed adjacent to one another on the substrate. Each FET includes a silicon-on-insulator (SOI) region, a gate electrode disposed on the SOI region, a source stressor, and a drain stressor disposed across from the source stressor relative to the gate electrode, wherein proximities of the source stressor and the drain stressor to a channel of a respective FET are substantially equal.

BACKGROUND

1. Technical Field

The present invention is related generally to complementarymetal-oxide-semiconductor (CMOS) field-effect transistors, and moreparticularly to CMOS with embedded source/drain stressors and CMOS withraised source/drain.

2. Discussion of Related Art

Complementary metal-oxide semiconductor (CMOS) field-effect transistors(FETs) are employed in almost every electronic circuit application, suchas signal processing, computing, and wireless communications. CMOS chipsin manufacturing comprise of planar thick-body devices on bulk Sisubstrates or on silicon-on-insulator (SOI) substrates. Thick-body FETson SOI substrates are also referred to as partially-depleted SOI (PDSOI)FETs. A CMOS chip consists n-channel FETs (NFETs), p-channel PFETs(PFETs), and other electronic devices, such as but not limited to,diodes, resistors, capacitors, and inductors.

In CMOS circuits/chips comprising of either bulk Si or PDSOI FETs,embedded SiGe (eSiGe) source/drain (SD) is used in PFETs for obtaininguniaxial compressive stress in the channel for mobility and performanceenhancement. The eSiGe SD is typically fabricated by (i) forming nitridedisposable sidewall spacers and nitride gate cap to encapsulate the PFETgates, and forming nitride disposable cover on the NFET gates, (ii)etching a recess in the PFET SD regions using reactive-ion etch (RIE),and (iii) epitaxially growing SiGe in the recessed PFET SD regions,selective to nitride and oxide. Therefore, the eSiGe SD is laterally adistance dPFET away from the gate edge. Since the gate defines thechannel region underneath it, the eSiGe SD stressor is, therefore, adistance dPFET away from the channel.

Embedded SiC source/drain (SD) leads to mobility and performanceenhancement when used in NFETs for obtaining uniaxial tensile stress inthe channel. The eSiC SD is typically fabricated using processes similarto eSiGe SD fabrication. That is, eSiC SD is fabricated by (i) formingnitride disposable sidewall spacers and nitride gate cap to encapsulatethe NFET gates, and forming nitride disposable cover on the PFET gates,(ii) etching a recess in the NFET SD regions using reactive-ion etch(RIE), and (iii) epitaxially growing SiC in the recessed NFET SDregions, selective to nitride and oxide. Therefore, similar to the eSiGecase, the eSiC SD stressor is a distance dNFET away from the channel.

The effectiveness of eSiC and eSiGe SD stressors in enhancing carriermobility and FET performance depends on the proximity (that is, thedistance dNFET or dPFET) of the stressors to the channel. A closerproximity, that is, smaller dNFET or dPFET, leads to improved stresscoupling to the channel, thereby leading to higher enhancement ofcarrier mobility and FET performance.

Conventional methods of fabrication, similar to the one noted above, canbe used for fabricating CMOS structures with NFETs having eSiC SD andPFETs having eSiGe SD. One can also reverse the sequence of eSiGe andeSiC SD formation with eSiC SD for NFETs being formed prior to eSiGe SDfor PFETs. Conventional methods of fabrication lead to CMOS structureswhere the proximity of the NFET and PFET embedded SD stressors to therespective channels is different. The embedded SD that is formed secondis further away from its channel than the one that is formed first.Therefore, in these CMOS structures, the effectiveness of the embeddedSD stressors will be more for the embedded SD that is formed first andwill be less for the embedded SD that is formed next.

Scaling down the gate length of both NFETs and PFETs in CMOS circuits toshorter dimensions leads to increased CMOS circuit speed. The entiresemiconductor industry has followed this since the advent of CMOS.However, detrimental short-channel effects lead to high off-stateleakage currents in CMOS devices with short gate lengths, therebyincreasing the power consumption. In case of extreme short-channeleffects, CMOS circuits fail to operate.

Narrow-body planar and non-planar FETs, such as, extremely-thin SOI(ETSOI) FETs, FinFETs, and trigates exhibit significantly superiorshort-channel characteristics compared to thick-body bulk Si and PDSOIFETs. These FET architectures are, therefore, attractive candidates forfuture-generation CMOS technology. Narrow-body FETs suffer from highseries resistance. This problem is alleviated by epitaxially formingraised source/drain (RSD). This structure can be formed using processsimilar to those used for forming embedded SD (with the generalexception of skipping the recess etch step). Similarly, one can formCMOS using thin-body FETs with SiC RSD for NFETs and SiGe RSD for PFETs.

Similar to the case of thick-body CMOS with embedded SD discussedearlier, eSiC and eSiGe RSD can also impart favorable tensile andcompressive stress to NFET and PFET channels, respectively. Also,similar to the thick-body CMOS case, it is also well known in literaturethat the effectiveness of eSiC and eSiGe RSD stressors in enhancingcarrier mobility and FET performance depends on the proximity (that is,the distance dNFET or dPFET) of the stressors to the channel. A closerproximity, that is, smaller dNFET or dPFET, leads to better stresscoupling to the channel, thereby leading to higher enhancement ofcarrier mobility and FET performance.

Therefore, a need exists for a method for the fabrication of embedded SDCMOS where the proximity of an embedded SD is substantially the same forboth NFETs and PFETs and for fabrication of thin-body FET CMOS with SiCRSD NFETs and SiGe RSD PFETs with the proximity of the RSD being low andsubstantially the same for both NFETs and PFETs.

BRIEF SUMMARY

According to an embodiment of the present disclosure, a complementarymetal-oxide semiconductor (CMOS) structure includes a substrate, aP-type field effect transistor (FET), and an N-type FET disposedadjacent to one another on the substrate. Each FET includes asilicon-on-insulator (SOI) region, a gate electrode disposed on the SOIregion, a source stressor, and a drain stressor disposed across from thesource stressor relative to the gate electrode, wherein proximities ofthe source stressor and the drain stressor to a channel of a respectiveFET are substantially equal.

According to an embodiment of the present disclosure, a method forforming a complementary metal-oxide semiconductor (CMOS) structure withequal proximity of source/drain (SD) stressors to channels of adjacentN-type and P-type field effect transistors (FETs) formed on a substrateincludes forming a spacer comprising a stack comprising a top nitridelayer, an oxide layer, and a lower nitride layer, wherein the lowernitride layer determines the proximity of the SD stressors to thechannel for the N-type and P-type FETs, forming a photoresist on theCMOS structure which covers the N-type FET and exposes the P-type FET,implanting ions in an exposed portion of the top nitride layer of theP-type FET, etching, selectively, an exposed portion of the top nitridelayer and a portion of the oxide layer below the exposed portion of thetop nitride layer, removing the photoresist to expose the spacer on theN-type FET, forming a nitride spacer on sidewalls of a gate electrode ofthe P-type FET, wherein remaining portions of the lower nitride layer onthe P-type FET are removed, forming the SD stressors of the P-type FETon opposite sides of the gate electrode, wherein a distance between theSD stressors and the P-type FET is controlled by the nitride spacer,depositing a dielectric layer over the CMOS structure, forming aphotoresist on the CMOS structure which covers the P-type FET andexposes the N-type FET, implanting ions in an exposed portion of the topnitride layer of the N-type FET, etching, selectively, an exposedportion of the top nitride layer and a portion of the oxide layer belowthe exposed portion of the top nitride layer, removing the photoresistto expose the spacer on the P-type FET, forming a nitride spacer onsidewalls of a gate electrode of the N-type FET, wherein remainingportions of the lower nitride layer on the N-type FET are removed,forming the SD stressors of the N-type FET on opposite sides of the gateelectrode, wherein a distance between the SD stressors and the N-typeFET is controlled by the nitride spacer, and removing the dielectriclayer from the P-type FET.

According to an embodiment of the present disclosure, a method forforming a complementary metal-oxide semiconductor (CMOS) structure withequal proximity of source/drain (SD) stressors to channels of adjacentN-type and P-type field effect transistors (FETs) formed on a substratecomprises forming a spacer comprising a stack comprising a top nitridelayer, an oxide layer, and a lower nitride layer, wherein the lowernitride layer determines the proximity of the SD stressors to thechannel for the N-type and P-type FETs, and forming, selectively, the SDstressors on opposite sides of gate electrodes of the N-type FET and theP-type FET, wherein the gate electrodes are formed on respectivesilicon-on-insulator layers.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Preferred embodiments of the present disclosure will be described belowin more detail, with reference to the accompanying drawings:

FIGS. 1A-L illustrate an exemplary method for fabricating an embedded SDCMOS according to an embodiment of the present disclosure; and

FIG. 2 illustrates an example of a thin-body FET CMOS according to anembodiment of the present disclosure.

DETAILED DESCRIPTION

According to an embodiment of the present disclosure, CMOS with embeddedsource/drain stressors and CMOS with raised source/drain may be formedhaving substantially equal distances dNFET or dPFET.

In FIGS. 1A-L the CMOS devices are denoted as FET-1 and FET-2. If FET-1is NFET, then FET-2 will be PFETs. Similarly, if FET-1 is PFET, thenFET-2 will be NFET.

Given a wafer, conventional CMOS processing up to reox spacer 102formation on a silicon-on-insulator (SOI) substrate 101 leads to athick-body PDSOI structure shown in FIG. 1A. An exemplary process forthe formation of the structure of FIG. 1A includes an active areadefinition using pad oxidation, pad nitride deposition, active arealithography, active area etch, resist strip, optional shallow trenchisolation (STI) liner deposition, STI oxide deposition, optional STIoxide densification anneal, and STI chemical mechanical polish (CMP);removal of any remaining STI oxide on active areas followed by removalof pad nitride; channel implants (also known as well implants) usinglithography, ion implantation, and resist strip (performed once forNFETs and once for PFETs); gate stack formation using pad oxide removal,oxidation to form the gate dielectric, poly Si deposition to form thegate electrode, oxidation to form poly Si screen oxide for gate ionimplants; optional gate pre-doping using photolithography, ionimplantation, and resist strip (performed once for NFETs and once forPFETS); nitride and oxide cap layer deposition (needed for selectiveepitaxy); gate definition using lithography, gate stack etch, and resiststrip; and oxidation to form reox spacer 102.

After the formation of reox spacer 102, a triple dielectric layer 103,such as, nitride/oxide/nitride, is deposited, leading to the structureshown in FIG. 1B. Photolithography is performed to cover FET-1 in aphotoresist 104 and expose FET-2. Ion implantation is performed on a toplayer of the exposed portion of the triple dielectric layer 103 tocreate implanted nitride top layer 105. The implantation step modifiesthe etch properties of the top nitride layer. For example, a BF₂implanted nitride can be etched by hydrofluoric acid while unimplantednitride does not etch in hydrofluoric acid. The wafer is etched in anetchant that can etch both implanted nitride and the oxide beneath itbut is selective to photoresist and unimplanted nitride, leading to thestructure shown in FIG. 1D. One such etchant is hydrofluoric acid.Nitride RIE is performed with the photoresist intact and the photoresist104 is then removed, leading to the structure shown in FIG. 1E. FET-2now has a nitride spacer with width d_(SP1). A width of an inner nitridelayer 103′ on FET-1 sidewall is also d_(SP1).

A recess is created in FET-2 SD regions and is followed by selectiveepitaxy of embedded SD stressor 105, leading to the structure shown inFIG. 1F. The embedded SD stressor 105 for FET-2 is denoted as SiX-2.SiX-2 may be formed of SiC when the FET-2 is NFET and formed of SiGewhen FET-2 is PFET. The proximity of SiX-2 SD to FET-2 channel isd_(FET2)=d_(SP1). The channel is formed directly below the respectivegate electrode in the SOI.

Having formed the embedded SD stressor 105 for FET-2, a singledielectric layer 106, such as, nitride, is deposited, leading to thestructure shown in FIG. 1G. Photolithography is then performed to coverFET-2 in a photoresist 107 and expose FET-1. Ion implantation performedon a top nitride layer on FET-1 to create implanted nitride top layer108, leading to the structure shown in FIG. 1H. The implantationmodifies the etch properties of the top nitride layer. For example, asmentioned herein, BF₂ implanted nitride can be etched by hydrofluoricacid while unimplanted nitride does not etch in hydrofluoric acid. Thewafers are etched in an etchant that can etch both implanted nitride andthe oxide beneath it but is selective to photoresist and unimplantednitride, leading to the structure shown in FIG. 1I. One such etchant ishydrofluoric acid. Nitride RIE is then performed with the photoresistintact and the photoresist 107 is then removed, leading to the structureshown in FIG. 1J. FET-1 now has a nitride spacer with width d_(SP1).

A recess is then created in FET-1 SD regions and is followed byselective epitaxy of an embedded SD stressor 109, leading to thestructure shown in FIG. 1K. The embedded SD stressor 109 for FET-1 isdenoted as SiX-1. SiX-1 109 may be formed of SiC if FET-1 is NFET andformed of SiGe if FET-1 is PFET. The nitride layers are removed using anitride etchant, leading to the structure shown in FIG. 1L. Theproximity of embedded SiX-1 109 SD to FET-1 channel (d_(FET1)) andembedded SiX-2 105 SD to FET-2 channel (d_(FET2)) are both substantiallyequal to d_(SP1).

Thin-body FETs can be similarly fabricated with a substantially equalproximity of the NFET and PFET raised source/drains (RSD) to therespective channels, leading to the thin-body field-effect transistorstructure shown in FIG. 2.

According to an embodiment of the present disclosure, a proximity of aRSD, e.g., 201, to a channel region directly below the gate electrode202 plays a role in thin-body field-effect transistors (FETs). As shownin FIG. 2, a SOI region 203 underneath a disposable nitride spacerremains thin after RSD epitaxy because the SOI region is covered bynitride during RSD epitaxy. This thin region of SOI, referred to as thesource/drain extension (SDE) region 204, is a source of seriesresistance for thin-body FETs. The resistance of a region is directlyproportional to its length. For thin-body FETs, the length of theresistive SDE region is the proximity of the RSD to the channel. Acloser proximity, that is, smaller dNFET or dPFET, leads to lower seriesresistance, thereby leading to higher FET drive current and improved FETperformance.

Having formed embedded SD or raised SD with equal proximity for bothNFETs and PFETs, one can follow conventional processing for chipfabrication.

Having described embodiments for a system and method of discovering anetwork topology from routing information, it is noted thatmodifications and variations can be made by persons skilled in the artin light of the above teachings. It is therefore to be understood thatchanges may be made in the particular embodiments of the inventiondisclosed which are within the scope and spirit of the invention asdefined by the appended claims. Having thus described the invention withthe details and particularity required by the patent laws, what isclaimed and desired protected by Letters Patent is set forth in theappended claims.

What is claimed is:
 1. A complementary metal-oxide semiconductor (CMOS)structure comprising: a substrate; and a P-type field effect transistor(FET) and an N-type FET disposed adjacent to one another on thesubstrate, each FET comprising, a silicon-on-insulator (SOI) region, agate electrode disposed on the SOI region, a source stressor, and adrain stressor disposed across from the source stressor relative to thegate electrode, wherein proximities of the source stressor and the drainstressor to a channel of a respective FET are substantially equal. 2.The CMOS structure of claim 1, wherein the source stressor and the drainstressor are formed in the SOI region.
 3. The CMOS structure of claim 1,wherein the source stressor and the drain stressor are formed on the SOIregion.
 4. The CMOS structure of claim 1, further comprising a shallowtrench isolation region disposed between the respective SOI regions ofthe P-type FET and the N-type FET.
 5. The CMOS structure of claim 1,wherein the channel is formed directly below the gate electrode.
 6. TheCMOS structure of claim 1, further comprising a spacer disposed betweeneach the gate electrode and the SOI region.
 7. A method for forming acomplementary metal-oxide semiconductor (CMOS) structure with equalproximity of source/drain (SD) stressors to channels of adjacent N-typeand P-type field effect transistors (FETs) formed on a substratecomprising: forming a spacer comprising a stack comprising a top nitridelayer, an oxide layer, and a lower nitride layer, wherein the lowernitride layer determines the proximity of the SD stressors to thechannel for the N-type and P-type FETs; forming a photoresist on theCMOS structure which covers the N-type FET and exposes the P-type FET;implanting ions in an exposed portion of the top nitride layer of theP-type FET; etching, selectively, an exposed portion of the top nitridelayer and a portion of the oxide layer below the exposed portion of thetop nitride layer; removing the photoresist to expose the spacer on theN-type FET; forming a nitride spacer on sidewalls of a gate electrode ofthe P-type FET, wherein remaining portions of the lower nitride layer onthe P-type FET are removed; forming the SD stressors of the P-type FETon opposite sides of the gate electrode, wherein a distance between theSD stressors and the P-type FET is controlled by the nitride spacer;depositing a dielectric layer over the CMOS structure; forming aphotoresist on the CMOS structure which covers the P-type FET andexposes the N-type FET; implanting ions in an exposed portion of the topnitride layer of the N-type FET; etching, selectively, an exposedportion of the top nitride layer and a portion of the oxide layer belowthe exposed portion of the top nitride layer; removing the photoresistto expose the spacer on the P-type FET; forming a nitride spacer onsidewalls of a gate electrode of the N-type FET, wherein remainingportions of the lower nitride layer on the N-type FET are removed;forming the SD stressors of the N-type FET on opposite sides of the gateelectrode, wherein a distance between the SD stressors and the N-typeFET is controlled by the nitride spacer; and removing the dielectriclayer from the P-type FET.
 8. The method of claim 7, wherein the SDstressors are forming in a silicon-on-insulator formed between the gateelectrode and the substrate.
 9. The method of claim 7, wherein the SDstressors are forming on a silicon-on-insulator formed between the gateelectrode and the substrate.
 10. The structure of claim 7, wherein thenitride spacer of the P-type and N-type FETs have substantially the samewidth.
 11. A method for forming a complementary metal-oxidesemiconductor (CMOS) structure with equal proximity of source/drain (SD)stressors to channels of adjacent N-type and P-type field effecttransistors (FETS) formed on a substrate comprising: forming a spacercomprising a stack comprising a top nitride layer, an oxide layer, and alower nitride layer, wherein the lower nitride layer determines theproximity of the SD stressors to the channel for the N-type and P-typeFETs; and forming, selectively, the SD stressors on opposite sides ofgate electrodes of the N-type FET and the P-type FET, wherein the gateelectrodes are formed on respective silicon-on-insulator layers.
 12. Themethod of claim 11, wherein the SD stressors are forming in asilicon-on-insulator formed between the gate electrode and thesubstrate.
 13. The method of claim 11, wherein the SD stressors areforming on a silicon-on-insulator formed between the gate electrode andthe substrate.